1. Field of the Invention
The present invention relates to a method of manufacturing a FLOTOX type EEPROM, and particularly to a method of forming a tunnel oxide film region.
2. Description of the Related Art
A method of manufacturing a FLOTOX (FLOating gate Tunnel Oxide) type EEPROM, according to a prior art is shown in FIGS. 1 through 4. A field oxide film 2 corresponding to a device or element isolation region is formed on a P type silicon substrate 1 by a LOCOS method. After a silicon nitride film employed in a LOCOS process has been removed, boron ions are selectively injected directly below the field oxide film 2 to form a channel stop region 3. Then, N type ions such as phosphorus ions are selectively injected and annealing is effected to form a tunneling implanted region 4 corresponding to an N type diffusion region formed with a tunnel oxide film (see FIG. 1).
Subsequently, the entire surface of the substrate, containing an exposed active region is oxidized about 30 nm to form a gate oxide film 5. A resist 6 is applied thereonto and thereafter subjected to resist patterning to form a tunnel window opening 7 on its corresponding tunneling implanted region 4 (see FIG. 2). The gate oxide film 5 in the tunnel window opening 7 is etched by a wet etching solution such as hydrofluoric acid to expose part of the tunneling implanted region 4, followed by removal of the resist 6.
Then, the entire surface of the substrate is oxidized about 10 nm to form a locally thin tunnel oxide film 9 having a thickness of about 10 nm and a re-oxidized gate oxide film 10 in a tunnel window region 8 (see FIG. 3). Thereafter, polysilicon is deposited and subjected to patterning to thereby form a floating gate electrode 11 so as to cover the tunnel window region 8 (see FIG. 4). Although not shown in the figure, a control gate electrode, wiring and the like are thereafter formed, thereby leading to completion of the FLOTOX type EEPROM.                Japanese Unexamined Patent Publication No. Sho 61(1986)-228672        Japanese Unexamined Patent Publication No. Hei 4(1992)-145666        Japanese Unexamined Patent Publication No. Hei 4(1992)-207084        Japanese Unexamined Patent Publication No. 2001-210730        
In the manufacturing method according to the prior art, however, the thick gate oxide film is processed using the wet etching solution in the tunnel window region etching process for the formation of the tunnel oxide film. Therefore, an opening in an actual tunnel window region becomes large as compared with a resist's pattern, thus leading to a large factor that inhibits a reduction in cell size.
When the opening of the tunnel window region is performed by a dry etching method, a problem arises in that etching damage which is a problem peculiar to dry etching, occurs, thereby deteriorating the quality of a subsequent tunnel oxide film. Further, a problem arises in that since the open area of the tunnel window region is small, the detection of an end point of dry etching becomes difficult.